The sheen of paint refers to how shiny it is.  For most indoor walls, people usually use the following guidelines. 

Semi-gloss paints are more stain resistant than their low-sheen counterparts.  Use these in high-traffic, high-mess areas like kitchens and bathrooms.  Eggshell and Satin paints are a little less glossy than semi-gloss and are a good option for those who dislike the look of shine on their walls.  Finally, a matte or flat finish is a perfect choice for a ceiling or a sophisticated look on walls that see little to no traffic.  One other rule of thumb is that the shinier the paint, the more imperfections it shows (brush marks, drywall imperfections, etc.).